This invention relates generally to a computed tomography (CT) system and more particularly to systems and methods for eliminating artifacts caused when using multiple x-ray sources.
Typically, in a CT system, an x-ray tube emits a fan-shaped x-ray beam or a cone-shaped x-ray beam toward a subject or object positioned on a table. The beam, after being attenuated by the subject, impinges upon a detector assembly. The intensity of the attenuated x-ray beam received at the detector assembly is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector assembly produces a separate electrical signal indicative of the attenuated x-ray beam received.
In known third generation CT systems, the x-ray source and the detector assembly are rotated on a gantry around the object to be imaged so that a gantry angle at which the fan-shaped or cone-shaped x-ray beam intersects the object constantly changes. The table supporting the subject may be advanced while the gantry is rotating around the object being imaged. Data representing the strength of the received x-ray beam at each of the detector elements is collected across a range of gantry angles. The data is ultimately reconstructed to form an image of the object.
It is advantageous to have large coverage in a z-direction for certain procedures. For example, large coverage allows for the collection of data in fewer gantry revolutions, which leads to a quicker acquisition time. Also, large coverage may allow an entire organ to be acquired in a single gantry revolution. The size of the coverage may be increased in the z-direction by increasing the width of the detector assembly. To overcome limitations caused by a cone-beam artifact for wide detector assemblies that use a single x-ray source, two or more x-ray sources may be displaced along the z-axis. When imaging, the x-ray sources alternately transmit x-rays. The x-ray beams from the x-ray sources may partially overlap each other when traveling through the subject, and therefore the x-ray flux varies along the z-axis. The combined image experiences noise artifacts due to the greater flux within the overlapping region. In addition, the patient is exposed to approximately twice the level of radiation within the overlapping region as compared to non-overlapping regions.